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ColBERT Reranker

Enhance search results with ColBERT's contextual reranking in LanceDB. Features efficient model deployment, device optimization, and flexible scoring options for vector, FTS, and hybrid search.

ColBERT Reranker

This reranker uses ColBERT model to rerank the search results. You can use this reranker by passing ColbertReranker() to the rerank() method.

Note: Supported query types: Hybrid, Vector, and FTS.

import tempfile
from pathlib import Path

import lancedb
from lancedb.embeddings import get_registry
from lancedb.pydantic import LanceModel, Vector
from lancedb.rerankers import ColbertReranker

embedder = get_registry().get("sentence-transformers").create()
db = lancedb.connect(str(Path(tempfile.mkdtemp()) / "colbert-rerank"))

class Schema(LanceModel):
    text: str = embedder.SourceField()
    vector: Vector(embedder.ndims()) = embedder.VectorField()

data = [
    {"text": "hello world"},
    {"text": "goodbye world"},
]
tbl = db.create_table("test", schema=Schema)
tbl.add(data)
reranker = ColbertReranker()

# Run vector search with a reranker
result = tbl.search("hello").rerank(reranker=reranker).to_list()

# Build the full-text index before either FTS or hybrid search.
tbl.create_fts_index("text", replace=True)

# Run FTS search with a reranker
result = tbl.search("hello", query_type="fts").rerank(reranker=reranker).to_list()

# Run hybrid search with a reranker
result = (
    tbl.search("hello", query_type="hybrid").rerank(reranker=reranker).to_list()
)

Accepted Arguments

Argument	Type	Default	Description
model_name	str	"colbert-ir/colbertv2.0"	The name of the reranker model to use.
column	str	"text"	The name of the column to use as input to the cross encoder model.
device	str	None	The device to use for the cross encoder model. If None, will use "cuda" if available, otherwise "cpu".
return_score	str	"relevance"	Options are "relevance" or "all". The type of score to return. If "relevance", will return only the _relevance_score column. If "all" is supported, will return relevance score along with the vector and/or fts scores depending on query type.

Supported Scores for each query type

You can specify the type of scores you want the reranker to return. The following are the supported scores for each query type:

Hybrid Search

return_score	Status	Description
relevance	✅ Supported	Results only have the _relevance_score column.
all	❌ Not Supported	Results have vector(_distance) and FTS(score) along with Hybrid Search score(_relevance_score).

Vector Search

return_score	Status	Description
relevance	✅ Supported	Results only have the _relevance_score column.
all	✅ Supported	Results have vector(_distance) along with Hybrid Search score(_relevance_score).

FTS Search

return_score	Status	Description
relevance	✅ Supported	Results only have the _relevance_score column.
all	✅ Supported	Results have FTS(score) along with Hybrid Search score(_relevance_score).